Feat/active cell wrapper

[sjhorn]

Context

For rare situations, you may want to render a different background for the active selection and then paint over that with the selected cell content. This wrapper enables that.

I encountered this when trying to have SwayzeTable look like the original VisiCalc which I am preparing as a demo of this great open source engine.

Approach

Introduced a wrapper similar to the header, body and box wrappers. Needed to position the wrapper to simplify the widget tree without having to implement a customer painter.

Focused on activeRect (Active cell Rectangle) for this wrapper, could be expanded to support selectedRect (Selection Rectangle) but wanted to keep the scope small for a pull request/feature.